1. Field of the Invention
This invention relates to a method and an apparatus for detecting the position of the level of molten metal, and particularly to, a method and an apparatus for detecting the position of the level of molten metal within a mold in operation of a continuous casting apparatus.
2. Description of the Related Art
The following methods of detecting the position of the level of molten metal:
1) Use of a float
A float is floated on a molten metal are known in the art surface and the position of this float is detected by a bar, a chain or the like.
2) Use of an optical (optoelectric conversion) technique
Since the brightness changes at a boundary between the molten metal surface and the container such as a mold, this boundary is measured by triangulation technique using for example, a sensor array, television cameras or the like.
3) Use of an ultrasonic wave
The distance to a surface of molten metal is measured by a time required for an ultrasonic wave irradiated on the surface of the molten metal to return an original position after reflected by the surface of the molten metal.
4) Use of a radioactive ray
A radioactive ray is transmitted through the molten metal in a diagonal direction, and the surface level of the molten metal is detected from the amount in attenuation of the radioactive ray during transmission through the molten metal.
5) Use of a immersed electrode
The level of the molten metal is detected by turning on or off of an electric circuit provided by the molten metal and an immersed electrode.
6) Use of a thermocouple
Several thermocouples are buried in the outer surface of the wall of the container of the molten metal, and the level of the molten metal is indirectly detected from the changing point in the temperature distribution measured by the thermocouples.
7) Use of electromagnetic induction
For example, in the Japanese Patent No. 55-16749, a coil which is long in the depth direction of the container is provided on the outer surface of the wall of the mold and connected to one side of an impedance bridge circuit. Since the temperature of the mold wall changes with the change of the level of the molten metal within the mold, resulting in change of the specific resistance of the mold wall, the level of the molten metal is detected from the change of the eddy current produced in the mold wall due to the change of the specific resistance of the mold wall.
The conventional methods for detecting the level of the molten metal, however, involve the following problems:
1) Use of a float
The float is subjected to corrosion by the high-temperature molten metal, and also the slug, molten metal or the like is sometimes adhered to the float, thereby changing the specific gravity of the float, resulting in the necessity of the calibration.
2) Use for optical (optoelectric conversion) technique
When a smoke, dust or the like is present or when the slug (which generally provides a low brightness and looks black) is floated on the surface of the molten metal, the measurement is difficult. Further when the optical sensor portion is soiled by this smoke or the like, or when the molten metal is at a high temperature and the light is refracted by the heat waves, the measurement may involve errors.
3) Use of an ultrasonic wave
When the molten metal is at a high temperature, the air is waved (changed in its density) by the heat, causing complicated refraction of sound to disable the measurement.
4) Use of a radioactive ray
There is a problem with safety, and the radiation source and the detector must be placed in a special space.
5) Use of an immersed electrode
The electrode is consumed greatly in the high-temperature molten metal and thus cannot be used for a long time.
6) Use of a thermocouple
Since the container for the high-temperature molten metal is made of firebricks, its heat conduction is poor. Thus, not only is the measurement delayed but also the detection precision is poor. Moreover, it is difficult to bury the thermocouples in the container wall, or to exchange a broken thermocouple with a new one.
7) Use of electromagnetic induction (Japanese Patent Publication No. 55-16749).
The temperature change at the molten metal surface is indirectly measured as the temperature change on the mold wall. However, the temperature change on the mold wall at the molten metal surface is less sharp, thus causing measurement errors.
Moreover, since the mold is cooled, it is more difficult to detect this temperature change as an impedance change, thus inevitably causing measurement errors.
Further, in a recently developed continuous casting apparatus using a belt drive type mold, as disclosed, for example, in Japanese Patent laid-open No. JP-A-60-152347, it is required to continuously detect the level of the molten metal within the mold during operation. However, any of the conventional methods as above-mentioned is difficult to be used for the purpose, because the surface of the molten metal exposed to the top of this type of mold is small and various auxiliary devices for operation of the mold are disposed on the outside of the mold, resulting in insufficient physical space.